Holiday detectors also known as porosity detectors, spark testers, porosimeters or pinhole detectors are employed in the non-destructive detection of apertures or thin points in protective coatings on surfaces of metal pipes and storage tank base plates susceptible to corrosion from fluid in contact with such surfaces. These detectors may be operated with a stationary power supply or a battery carried by the detector.
For the present invention relevant holiday testers employ for the contact element a brush or sponge into which is directed and electrolytic solution, so that as the sponge or brush sweeps across the protective coating and comes into the vicinity of a holiday, electrolytic fluid can flow into the holiday and to the electrically grounded metal of the base plate and close the circuit, with the resulting spark or current flow that is registered by the controller.
Some holiday detection in liquid petroleum storage tanks has been done manually by persons who enter the tank and move a sponge apparatus along the surface of the base plate. This must be followed up directly by marking the location of the holiday with an ink stamp or other marking indicator suitable for material environment, and subsequent repair of the coating in a separate operation.
The environment inside liquid petroleum storage tank is both unpleasant and dangerous for humans because of the residual liquid petroleum fumes that are inhospitable for breathing and present a danger of fire and explosion.
Holiday testing apparatus can be used for example with a low-voltage wet sponge test for determining the existence of discontinuities in coating films having a total thickness of 0.5 mm or less. High-voltage spark test equipment is generally used for determining the existence of discontinuities in coating films having a total thickness of greater than 0.5 mm.
The fluid media, water or electrolytic fluid in the wet sponge fills the voids in the surface to be tested and allows low current to flow into any holidays in the test area. The current moves through the holiday and into the conductive substrate which activates a horn, light or other output to indicate the presence of the holiday.
The prior art includes manual holiday detection by humans carrying and operating holiday detection apparatus, and holiday detection apparatus carried by vehicles on tracks or within a pipe where the vehicle moves only in a straight path along which there are no obstacles to movement and thus no need to identify and avoid obstacles. Thus, conventional prior art holiday detection vehicles travel involve no concern or possibility for the vehicle to be turning, zigzagging, or even following parallel and/or overlapping paths.
The following U.S. patents disclose some of the above-mentioned prior art concepts and are incorporated herein by reference: U.S. Pat. Nos. 6,832,183, 3,106,677, 4,946,570 and 7,285,203.